AST Blood Test: What Your Result Says About Your Liver and Muscles

What is AST?

AST (aspartate aminotransferase) is an enzyme that moves nitrogen-containing groups between molecules inside your cells, one of the steps in turning amino acids into energy. It is found in the liver, skeletal muscle, heart, kidneys and red blood cells. A small amount in your blood is normal. More appears when those cells are injured or under strain. Older reports call it SGOT (serum glutamic-oxaloacetic transaminase).

ALT is concentrated in the liver, while AST is spread across several tissues, so AST is less liver-specific on its own. It also clears faster (a half-life of about 17 hours, against roughly 2 days for ALT), so the two can move at different speeds after the same event.

What does an AST blood test show?

An AST blood test shows whether cells that contain AST are being damaged faster than usual, most often in the liver or the muscles. It sits within a liver function test, but the number alone can't tell you which tissue it came from.

• Liver. AST rises with fatty liver disease, alcohol, viral hepatitis and some medicines. ALT usually rises with it.
• Muscle. Hard or unfamiliar exercise or a muscle injury can push AST up while your liver is fine. Creatine kinase is the marker that confirms a muscle source.
• Red blood cells. If red cells break open in the tube after collection (haemolysis), AST leaks out and reads falsely high.

Liver problems are usually silent for years. Fatty liver alone was estimated to affect 5.5 million Australians in 2019 (Adams et al., 2020). Normal does not always mean optimal. An AST near the top of the range, with ALT climbing beside it and a growing waistline, is worth acting on early.

AST normal range

For adults, AST is usually under 40 U/L.

Labs set their own ranges, and some use a lower upper limit for women, so check yours. Our optimal range is where we like AST to sit for long-term liver health, and it isn't a diagnostic line. Between 30 and 40 U/L, we look at your ALT, GGT, recent training and alcohol before reading much into it.

What does high AST mean?

A high AST means cells containing AST have been damaged recently, and in active, healthy people the most common reason is hard exercise. If ALT is high too and you haven't trained hard, the liver is the likelier source, and worth checking with your GP.

The usual suspects in a well adult:

• Hard or new exercise. Heavy weights, long runs or any training your muscles aren't used to. In a study of healthy men who didn't normally lift weights, one hour of weightlifting raised AST and ALT, and they stayed raised for at least 7 days (Pettersson et al., 2008).
• Alcohol. A big night or regular drinking above the guidelines.
• Medicines and supplements. Statins, high-dose paracetamol, some antibiotics, and herbal or bodybuilding products such as concentrated green tea extract.

Medical causes a doctor will consider:

• Fatty liver disease, now called MASLD (metabolic dysfunction-associated steatotic liver disease). ALT is usually higher than AST.
• Alcohol-related liver disease. AST is often more than twice ALT.
• Viral hepatitis (hepatitis B or C), and less often autoimmune liver disease or iron overload (haemochromatosis).
• An underactive thyroid or a muscle condition, which can raise AST and creatine kinase together.
• Coeliac disease, which can cause a mild, unexplained rise in liver enzymes.

One AST of 45 U/L after a heavy training week is very different from an AST of 45 U/L that has crept up over three tests with ALT alongside it.

What does low AST mean?

Low AST is rarely meaningful and isn't treated. AST relies on vitamin B6 to work, so a very low result can occasionally reflect low B6, and levels are often low in people on kidney dialysis.

What can affect your AST result

Recent exercise skews AST more than anything else. Things to plan for:

• Do you need to fast? No. You'll fast overnight for the Baseline's glucose marker, and that doesn't change AST.
• Training. Try to skip hard or unfamiliar training for 2 to 3 days before your test. Walking is fine. After something big, such as a marathon, give it about a week.
• Alcohol. Aim for no alcohol in the 2 to 3 days before.
• Medicines. Keep taking anything prescribed, and tell your practitioner about herbal products too.
• Recent illness. Viral infections can raise liver enzymes for a few weeks.

How AST fits with your other results

AST makes most sense next to ALT, and the AST/ALT ratio (also called the De Ritis ratio) helps show where a rise is coming from (Botros and Sikaris, 2013).

How to support healthy AST levels

If your AST is high from training, there is nothing to fix: it settles within about a week. If your liver is the source, the steps that lower liver fat bring AST down.

• Drink less, with alcohol-free days. The national guideline is a ceiling of 10 standard drinks a week, and less is better for your liver.
• Reduce weight around your middle, if you carry it there. Losing 5 to 10% of your body weight can lower liver fat and bring liver enzymes down.
• Eat Mediterranean-style. Build meals on vegetables, legumes, wholegrains, fish and olive oil, and cut back on sugary drinks, which add to liver fat.
• Train consistently and build up gradually. Regular moderate exercise reduces liver fat even without weight loss, and ramping up slowly limits big AST spikes.
• Check before you start supplements. Some herbal and bodybuilding products can injure the liver.

When to see your GP

See your GP promptly if a high AST comes with yellow eyes or skin, dark urine, pale stools, pain under your right ribs or unusual tiredness. Seek urgent care if you have dark brown, cola-coloured urine with severe muscle pain or weakness after exercise, as this can be rhabdomyolysis (serious muscle breakdown). For chest pain, call 000.

Also book in if your AST is above about 45 U/L on a repeat test after a quiet week, if ALT is high as well, or if it keeps rising. An AST above about 150 U/L needs prompt review, and above 500 U/L needs same-day medical attention.

Testing AST with Vively

AST is included in Vively's Baseline Health Check with ALT, GGT, ALP and bilirubin, so you can tell a liver signal from a muscle one. The exact markers in your Baseline can depend on the referring doctor's review.

Sign-up is online and our GP network handles the referral. You give one blood sample at an accredited partner collection centre (there are 4,000+), and results reach the app, usually within a few days, set against optimal ranges. Your practitioner explains them, you get a personalised plan, and a retest shows whether AST has settled. If it's up because of a big leg session two days before, we'll tell you. Creatine kinase isn't in the Baseline, so if muscle looks like the source, your practitioner can explain how to get it checked through your GP.

Check your AST and liver markers with the Baseline Health Check.

Frequently asked questions

Do I need to fast before an AST test?

No. Food has little effect on AST. Vively's Baseline asks you to fast overnight for its glucose and insulin markers, and that won't change your AST. Recent exercise matters far more, so try to avoid heavy or unfamiliar training for 2 to 3 days before your blood test.

Can exercise raise AST?

Yes. Muscle contains AST, so hard or unfamiliar exercise can push it above the lab range. In one study of men new to weights, AST and ALT stayed raised for at least a week after a single session. If you trained hard before your test, retest after a lighter week.

Is an AST of 50 U/L high?

It's above most labs' upper limit of 40 U/L, but only mildly. After hard training, that is common and usually harmless. If you haven't trained hard, your ALT or GGT is also high, or it stays high on a repeat test after a quiet week, see your GP to find the cause.

What is the difference between AST and ALT?

Both are enzymes that leak into the blood when cells are damaged. ALT is found mostly in the liver, so it is the more liver-specific marker. AST is also found in muscle, heart and red blood cells. Reading them together, including the AST/ALT ratio, shows whether a rise is more likely liver or muscle.

Can I get an AST test through Medicare?

It can be. Many blood tests, including AST, can be Medicare-funded when a doctor orders them for a clinical reason. With Vively, we arrange the referral through our GP network as part of your Baseline Health Check, so you can test without seeing a GP first.

References

• Royal College of Pathologists of Australasia (RCPA). RCPA Manual: Aspartate aminotransferase.
• Pathology Tests Explained. Aspartate aminotransferase (AST). https://www.pathologytestsexplained.org.au
• Gastroenterological Society of Australia (GESA). Understanding Liver Tests. 2024 update.
• Botros M, Sikaris KA. The De Ritis ratio: the test of time. Clinical Biochemist Reviews. 2013;34(3):117-130. https://pmc.ncbi.nlm.nih.gov/articles/PMC3866949/
• Pettersson J, Hindorf U, Persson P, et al. Muscular exercise can cause highly pathological liver function tests in healthy men. British Journal of Clinical Pharmacology. 2008;65(2):253-259.
• Adams LA, Roberts SK, Strasser SI, et al. Nonalcoholic fatty liver disease burden: Australia, 2019-2030. Journal of Gastroenterology and Hepatology. 2020.
• McGill MR. The past and present of serum aminotransferases and the future of liver injury biomarkers. EXCLI Journal. 2016;15:817-828.
• National Health and Medical Research Council. Australian guidelines to reduce health risks from drinking alcohol. 2020.